The taxiing of aircraft while on the ground is conventionally performed using thrust provided by the engines of the aircraft. However, it can be desirable to provide thrust through a wheel drive taxi system; in other words, an aircraft with landing gear incorporating drive motors, which drive the wheels to move the aircraft while it is on the ground.
Control of movement of an aircraft while on the ground using the aircraft's engines is achieved by use of the engine thrust controller. In known wheel drive taxi systems, the equivalent control is achieved using a torque controller, with which the pilot (or other person controlling the aircraft) commands a percentage of available torque (i.e. turning force) to the drive motors.
However, when stationary, the torque required for movement of the aircraft to begin can vary dependent upon the operating conditions. This is because the “breakaway resistance” of the aircraft will vary dependent upon various factors, such as the aircraft mass (which varies depending for different loads), centre of gravity, weather conditions, runway conditions (e.g. dry, wet, icy, sandy, rough, smooth), and any slope on which the aircraft is positioned, for example. When the torque is not sufficient to initiation movement of the aircraft, damage to the drive motors may be caused. If on the other hand a large torque is commanded in order to overcome a potentially large breakaway resistance, this can lead to undesirable “jerking” of the aircraft as movement is initiated.
The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide improved methods and control systems for controlling a drive system of an aircraft.